I tried a couple of system restores from before thr problem but for some reason the restores would not complete.
I tried your suggestions re. IPCONFIG and I can now download attachments as normal. However I didn't attemp to download immediately prior to making the IP configuration changes so don't know whether the problem was rectified at the mail server end or whether the config changes cured the snag.
Either way I'm very grateful for your help, it's very much appreciated.